Generally in a W-CDMA (Wideband-Code Division Multiple Access) system, a plurality of mobile terminals share a same radio frequency by code division multiplexing, and communicate in a radio cell area covered by one radio base station.
In this W-CDMA system, mutual interference by communication radio waves between mobile terminal increases as the number of mobile terminals accommodated and communicating in a same radio cell area increases. Therefore it is known that the number of mobile terminals (capacity) accommodated in a cell and the communication quality are in a trade-off relationship (see Non-patent Document 1 herein below).
As a consequence, in order to insure the capacity of an entire system and the communication quality for each user, it is important to perform appropriate controls, such as controlling the transmission power so that the communication power between the mobile terminal and a base station reaches the minimum according to the line status, or shutting communication OFF in an outside service area (“outside area”) if the mobile terminal communicating near a boundary of the radio cell area moves outside the cell area.
The mobile terminal is also demanded to perform various judgments related to a signal level of receive signals, and judge whether the mobile terminal is in an “outside area” or an “inside area” based on the result.
3GPP (3rd Generation Partnership Project), which specifies the radio standards for W-CDMA, defines three types of signal measurements as shown below (e.g. see Non-patent Document 2, Chapter 5.1 herein below).
(1) CPICH_RSCP (Common Pilot Channel_Received Signal Code Power): receive code power of the Primary CPICH (Primary pilot signal) measured by the mobile terminal,
(2) RSSI (Received Signal Strength Indicator): total power in a receive band measured by the mobile terminal, including both desired waves and interference waves, and
(3) CPICH_Ec/NO (Energy per chip/Noise Power): ratio of power per CPICH chip and total power per chip measured by the mobile terminal.
FIG. 20(A) shows a configuration example of a conventional W-CDMA mobile terminal (single antenna receiving) using these three types of signals.
A mobile terminal 300 has a receive antenna 311, an AGC (Automatic Gain Control) amplifier section 312, a first and second correlated value calculation sections 313-1 and 313-2, a storage memory 314, and a level measurement section 315.
A receive signal received by the receive antenna 311 is amplified to an appropriate signal level by the AGC section 312, and is input to the first and second correlated value calculation sections 313-1 and 313-2.
The second correlated value calculation section 313-2 calculates a correlated value using a diffusion code assigned to the receive signal. A necessary signal processing is performed on the output, and the receive signal is decoded.
The first correlated value calculation section 313-1, on the other hand, calculates a correlated value using a diffusion code assigned to a code channel which performs level measurement. This output is CPICH_RSCP+NO (NO is noise power). This value is stored in the storage memory 314 once.
In the AGC section 312, the control voltage thereof is in proportion to the signal level (RSSI) of the receive signal, so RSSI can be calculated. The calculated RSSI is also stored in the storage memory 314 once.
In the level measurement section 315, RSSI and CPICH_RSCP+NO are read from the storage memory 315, CPICH_RSCP is determined by subtracting the noise power NO from CPICH_RSCP+NO, and CPICH_Ec/NO is calculated by dividing CPICH_RSCP by RSSI.
The level measurement section 315 outputs (reports) the calculated CPICH_RSCP, CPICH-Ec/NO, and the RSSI read from the storage memory 314, to a higher layer. These three types of levels are used for Judging the “outside area” and “inside area” of the mobile terminal.
Definitions of the port on these three types of levels are specified in Non-patent Document 2, Chapter 5.1 herein below, but there is no statement on a particular specification considering receive diversity.
FIG. 20(B) shows a configuration example of a W-CDMA mobile terminal having a receive diversity function which could be implemented based on the configuration in FIG. 20(A).
A mobile terminal 330 has a first and second receive antennas 331-1 and 331-2, a first and second AGC sections 332-1 and 332-2, a first to third correlated value calculation sections 333-1 to 333-3, a combining section 334, a storage memory 335 and a level measurement section 336.
The receive signals received by the first and second receive antennas 331-1 and 331-2 are input to the first and second correlated value calculation sections 333-1 and 333-2 via the first and second AGC sections 332-1 and 332-2 respectively.
The first and second correlated value calculation sections 333-1 and 333-2 calculate correlated values (receive signals) respectively using diffusion codes, and the combining section 334 combines the outputs. Details on combining receive signals is stated in Non-patent Document 3, Chapter 7.2 herein below, for example. A composite signal is decoded by the decoding processing in subsequent stages, just like FIG. 20(A).
The second AGC section 332-2, on the other hand, outputs RSSI in the same way based on the control information. The receive signal that is output from the second AGC section 332-2 is also input to the third correlated value detection section 333-3, and RSCP+NO is output from the third correlated value detection section 333-3 in the same way as FIG. 20(A).
In the storage memory 335, RSCP+NO and RSSI, with respect to the receive signal received by the second receive antenna 331-1, are stored. The level measurement section 336 performs operation based on each value stored in the storage memory 335, and reports RSSI, CPICH_RSCP and CPICH_Ec/NO to a higher layer.    Non-patent Document 1: Ohno, Adachi: “Uplink capacity and transmission power of DS-CDMA”, Shingakuron B-II, VOL. J79-B-II, pp. 17-25, 1996-01    Non-patent Document 2: 3GPP TS 25.215V7.0.0 (2006-03): “Physical layer—Measurement (FDD)”.    Non-patent Document 3: 3GPP TS 25.101V7.3.0 (2006-03): “User equipment (UE) ratio transmission and reception (FDD)”.